Generate smooth normals when normals are missing

[nithinp7]

Previously we did not always generate normals when they were missing. Presumably Unity was generating normals for us in that case but potentially gets confused with the model transform we give our tiles. In particular, Unity and glTF have opposite handedness - while the conversion between the two is still possible in the form of a matrix, Unity only provides us the ability to individually set translation, rotation, and scale. The only workaround then, was to invert the change-of-handedness matrix to make it a valid rotation matrix, and use a negative scale so the handedness would be corrected in the final matrix.

Unfortunately, it seems like Unity may have been using the rotation without the scale when auto-generating normals for our tiles (after all one would think scale shouldn't affect rotation!). After this change, we always generate smooth normals in Cesium Native when normals are missing - so it is no longer an option on the tileset.

[kring]

Thanks @nithinp7!